There are many applications where it is desirable to accurately transfer tiny discrete liquid droplets from a source to a work piece. Examples include the production of DNA/protein micro-arrays and the dispensing of liquid metals in the fabrication of electrical micro-switches. One approach employs the so-called pin transfer method. See U.S. Pat. No. 6,579,499 granted Jun. 17, 2003 to Kennedy et al. In another approach a minute amount of liquid can be passed through a micro-nozzle. As the liquid protrudes from an orifice in the micro-nozzle, surface tension causes a tiny droplet to be formed. By way of example, the droplet may have a volume of one micro-liter and the nozzle orifice may have a radius of five hundred microns. The droplet can touch the surface of the work piece and then the nozzle and the work piece can be pulled apart to transfer the droplet to the work piece. Transfer of tiny droplets usually will not occur solely on the basis of gravity because it is insufficient to overcome the forces associated with wetting of the liquid to the surface of the nozzle.
The efficiency of the liquid droplet transfer utilizing a micro-nozzle depends upon many factors, including the droplet size, the nozzle orifice shape, the contact angle and the wettability differences between the nozzle surface and the work piece surface. It is desirable to efficiently transfer a predetermined droplet size from the nozzle.
The dispensing of liquid metals in the fabrication of micro-switches is currently achieved using a solitary nozzle or a pluarality of individual nozzles. The diameter of the nozzle orifice and the wettability of the nozzle surface is tailored to control the size of the desired liquid droplet. Both the process of fabricating such nozzles and altering their wettability, either by modifying the nozzle surface chemistry or nozzle surface roughness, can be very costly, particularly when the size of the droplet decreases and the required number of nozzles increases. Currently micro-nozzles are commercially available from the Lee Company (http://www.theleeco.com) that are made of brass alloy. In many cases these are unsuitable for dispensing liquid metal, such as Mercury, as this liquid metal can be contaminated from the Copper contained in the brass alloy.